1. Field of the Invention
This invention relates to compositions and methods for preparing polyurethane foams. In one aspect, the invention relates to compositions and methods for preparing water blown polyurethane spray foams. In another aspect, the invention relates to a method of applying water blown polyurethane foams.
2. Description of the Related Art
Building owners have used spray polyurethane foam as a roofing, insulation, and sealing product for many years. However, environmental concerns have prevented the continued use of conventional spray polyurethane foam formulations. In particular, the blowing agents used in conventional formulations to form spray polyurethane foams have come under increased regulation and in many cases, outright prohibition.
The Montreal Protocol of 1987 set forth progressive stages of change aimed at protecting the earth's upper ozone layer by reducing the amount of chlorine in the atmosphere. A significant portion of the chlorine is attributed to chlorinated blowing agents that are used in polyurethane foams. The United States Environmental Protection Agency (EPA) periodically approves new blowing agents containing lower amounts of chlorine and discontinues (bans) previously approved blowing agents.
As part of this protocol on Jan. 1, 1993, the polyurethane industry was required to change from the conventional blowing agent CFC-11 to HCFC-141b. The change from CFC-11 to HCFC-141b was not excessively difficult for the polyurethane industry because there were several commercial manufacturers of HCFC-141b and its cost was only slightly greater than CFC-11.
Currently, the polyurethane industry is being required to change from HCFC-141b. The suggested blowing agents are now HFC-245fa, hydrocarbons (pentanes) and others. This transition is much more of a challenge in part because HFC-245fa boils at approximately 60° F. compared to HCFC-141b boiling point of approximately 89° F. Since the 60° F. boiling point is below most ambient conditions, HFC-245fa creates pressure problems for materials shipped in the steel or plastic containers (drums and totes) that have commonly been used in the past with CFC-11 and HCFC-141b systems. Most spray polyurethane system formulators have attempted to use HFC-245fa and water as co-blowing agents to keep from having to use pressure cylinders to store and ship product.
In addition to pressure problems, HFC-245fa costs more than four times HCFC-141b in the United States. There is only one supplier of HFC-245fa and only one supplier of that manufacturer's supplier of base material with which to make the HFC-245fa. The additional cost represents a much higher cash flow requirement for most small business formulators to purchase needed amounts of the HFC-245fa. For the spray polyurethane foam roofing industry, the additional cost adversely affects the competitiveness of the spray polyurethane foam versus other roofing products on a cost basis.
Even more of a challenge is the attempt to use hydrocarbons. These compounds are highly flammable and have specific explosive properties. This is not to say hydrocarbons cannot be used to produce spray and pour polyurethane and polyisocyanurate foam systems, but significant expenditures in equipment and training must accompany any attempt by the contractor or OEM manufacturer to be able to use these compounds. In the sprayfoam industry, most systems suppliers and contractors have chosen not to work with these materials due to their hazardous nature and potential downstream liability.
The use of water in polyurethane foam forming compositions is not new. In fact, almost all polyurethane foam formulations have some water content either purposely added or as a minor trace component in the raw materials used. Water becomes a blowing agent by reacting with the isocyanate in the foam system. This reaction produces carbon dioxide as a by-product. The carbon dioxide is a gas and expands due to the heat of reaction of the foam system. The blowing agent is required to create the cellular nature of the foam structure and to help control the applied density of the foam.
Spray and pour polyurethane foam systems have traditionally minimized the use of water as a blowing agent due to adequate supplies of CFC or HCFC foaming agents being available at reasonable cost. The high cost of HFC-245fa and technical limitations associated with HFC-245fa and other potential blowing agents have initiated more efforts in the use of water. Using water, however, in place of HCFC or HFC blowing agents is not straight forward.
Water is a reactive blowing agent in polyurethane foam systems whereas CFC, HCFC, and HFC compounds are not reactive. Because water consumes a large amount of isocyanate (about 9 pounds of water will react with 134 pounds of isocyanate), a system is limited on the amount of water that can be used. In polyurethane foam systems, the isocyanate reacts with the polyol components to form the polymer structure. The isocyanate reacts with the water to cause the foam matrix. The isocyanate and the components that react with the isocyanate must be chemically, stoichiometrically, balanced to assure a complete reaction of the components. In practice, polyurethane systems are formulated with a small excess of isocyanate to assure complete reaction. This parameter is referred to as isocyanate index or simply “NCO index.” Commonly, polyurethane foams are designed in a NCO index range of 1.05–1.20. The NCO index is fixed and designed into the liquid system when it is produced. It is not readily controllable in the end-use application. The amount of isocyanate in spray polyurethane foam formulations is normally limited in use by industry standard application equipment which is a 1:1 (A:B) by volume ratio.
One attempt to provide a water blown polyurethane foam forming composition is found in U.S. Patent Application Publication No. U.S. 2002/0040122A1 to Mirasol, et al., now abandoned. Mirasol stated that the use of conventional Mannich polyols in water blown systems could cause equipment failure and other processing difficulties. Mirasol also stated that foams prepared using conventional Mannich polyols and water as a blowing agent can have coarse cell structure, rough skin surface, poor dimensional stability, poor flame retardancy, and poor substrate adhesion. Additionally, Mirasol stated that the combination of water and Mannich polyols could create formulations which have too high viscosity and could cause problems in some kinds of foam making equipment. In part, Mirasol recognized that a disadvantage of water as a blowing agent in polyol formulations is that water does not reduce the viscosity of Mannich polyols as effectively as halocarbon blowing agents. In an attempt to overcome these problems, Mirasol provided a B-side component that included an ultra low viscosity Mannich polyol having a viscosity of less than 3,500 centipoise (cps) at 25° C., along with a second polyol that included Mannich polyols having viscosity greater than 3,500 cps at 25° C., Novolac polyols, and other aromatic group containing polyols. Mirasol provided for only minor amounts of other polyols in its formulation. In general, the low viscoscity Mannich polyols have the characteristic of producing foams that are much slower to cure and reach adequate firmness. In spray formulations these characteristics are detrimental during application.
There remains a need for a polyurethane foam forming composition that does not rely on a halocarbon blowing agent. Additionally, there is a need for a water blown polyurethane spray foam having equivalent or better properties compared to conventional halocarbon blown foams. Further, there is a need for a polyurethane foam formulation that can use more readily available polyols, including readily available, conventional Mannich polyols, yet still produce superior polyurethane foam when using water as a blowing agent.